Cognitive aspects of mental activity during sleep.

Upon nighttime experimental awakening of 27 subjects in four sleep conditions (sleep onset early; sleep onset late; Stage 2; and rapid eye movement, REM, sleep), 108 dream reports and their association reports were collected. Dream reports were analyzed for length (temporal units) and content categories (continuity; implausibility; presence of the dreamer [i.e., "the self"], a setting, characters). Associations were classified as episodic, abstract self-referred, and semantic memories. The two sets of results tend to show a basic homogeneity among mentation reports in the four sleep conditions considered. These findings are interpreted as supporting the hypothesis that the same cognitive mechanisms operate, at different levels of engagement, in dream generation rather than the hypothesis of multiple dream-generation systems dependent upon the physiological characteristics of the various sleep stages.     

Memory performance after arousal from different sleep stages.

Learning material was presented to independent groups of subjects either after arousal from non-Rapid Eye Movement (non-REM) sleep, after arousal from REM sleep, or under conditions of no prior sleep. Measures of immediate and subsequent free recall were taken. Memory performance was found to be impaired where learning took place after non-REM arousal. This was manifest in the number of categories recalled, over both immediate and subsequent recall, and in the number of items recalled per category over subsequent recall. It was suggested that the memory performance decrement after non-REM arousal may be understood in terms of a retrieval deficit as well as a coding deficit. It is possible that the former is consequent upon a lower general level of arousal, whereas the latter is specific to memory.     

Memory consolidation during sleep: interactive effects of sleep stages and HPA regulation

Sleep is critically involved in the consolidation of previously acquired memory traces. However, nocturnal sleep is not uniform but is subject to distinct changes in electrophysiological and neuroendocrine activity. Specifically, the first half of the night is dominated by slow wave sleep (SWS), whereas rapid eye movement (REM) sleep prevails in the second half. Concomitantly, hypothalamo-pituitary-adrenal (HPA) activity as indicated by cortisol release is suppressed to a minimum during early sleep, while drastically increasing during late sleep. We have shown that the different sleep stages and the concomitant glucocorticoid release are interactively involved in the consolidation of different types of memories. SWS-rich early sleep has been demonstrated to benefit mainly the consolidation of hippocampus-dependent declarative memories (i.e. facts and episodes). In contrast, REM sleep-rich late sleep was shown to improve in particular emotional memories involving amygdalar function, as well as procedural memories (for skills) not depending on hippocampal or amygdalar function. Enhancing plasma glucocorticoid concentrations during SWS-rich early sleep counteracted hippocampus-dependent declarative memory consolidation, but did not affect hippocampus-independent procedural memory. Preventing the increase in cortisol during late REM sleep-rich sleep by administration of metyrapone impaired hippocampus-dependent declarative memory but enhanced amygdala-dependent emotional aspects of memory. The data underscore the importance of pituitary-adrenal inhibition during early SWS-rich sleep for efficient consolidation of declarative memory. The increase in cortisol release during late REM sleep-rich sleep may counteract an overshooting consolidation of emotional memories.     

Additive factors and stages of mental processes in task networks

To perform a task a subject executes mental processes. An experimental manipulation, such as a change in stimulus intensity, is said to selectively influence a process if it changes the duration of that process leaving other process durations unchanged. For random process durations a definition of a factor selectively influencing a process by increments is given in terms of stochastic dominance (also called “the usual stochastic order”). A technique for analyzing reaction times, Sternberg’s Additive Factor Method, assumes all the processes are in series. When all processes are in series, each process is called a stage. With the Additive Factor Method, if two experimental factors selectively influence two different stages by increments, the factors will have additive effects on reaction time. An assumption of the Additive Factor Method is that if two experimental factors interact, then they influence the same stage. We consider sets of processes in which some pairs of processes are sequential and some are concurrent (i.e., the processes are partially ordered). We propose a natural definition of a stage for such sets of processes. For partially ordered processes, with our definition of a stage, if two experimental factors selectively influence two different processes by increments, each within a different stage, then the factors have additive effects. If each process selectively influenced by increments is in the same stage, then an interaction is possible, although not inevitable.     

Differences in dimensionality of electroencephalogram during awake and deeper sleep stages

The nonlinear dynamical systems theory provides some tools for the analysis of electroencephalogram (EEG) at different sleep stages. Its use could allow the automatic monitoring of the states of the sleep and it would also contribute an explanatory level of the differences between stages. The goal of the present paper is to address this type of analysis, focusing on the most different stages. Estimations of dimensionality were compared when six subjects were awake and in a deep sleep stage. Greater dimensionality involves more complexity because the system receives more external influences. If this dimensionality is maximum, we can consider that the time series is a noisy one. A smaller dimensionality involves lower complexity because the system receives fewer inputs. We hypothesized that we would find greater dimensionality when subjects were awake than in a deep sleep stage. Results show a noisy time series during the awake stage, whereas in the sleep stage, dimensionality is smaller, confirming our hypothesis. This result is similar to the findings reached previously by other authors.     

Dynamics of mental activity

Motivated by neuronal modeling, our development of the mathematical foundations of consciousness in [W. Miranker, G. Zuckerman, Mathematical Foundations of Consciousness, J. Appl. Logic (2009)] (M-Z) was characterized by an axiomatic theory for consciousness operators that acted on the collection of all sets. Consciousness itself was modeled as emanating from the action of such operators on the labeled decoration of a graph, the latter set theoretic construct given the characterization of experience. Since mental activity (conscious and unconscious) is a time dependent process, we herein develop a discrete time dependent version of the theory. Specification of the relevant mental dynamics illuminates and expands the development of the mathematical framework in (M-Z) upon which our study of consciousness rests. This framework is an abstraction of neural net modeling.We review the Aczel theory for decorating labeled graphs, in particular that theory's application to the (M-Z) foundations. The relevant neuronal modeling concepts and terminology are also reviewed. A number of examples are presented. Then an extension of our considerations from graphs to multigraphs is made, since the latter represent a more accurate model of neuronal circuit connectivity. The dynamics are crafted for non-well-founded constructs by development of a hierarchy of systems, starting with the McCulloch–Pitts neuronal voltage input–output relations and building to a dynamics for the cognitive notions of memes and themata; these latter corresponding to aspects of decorations of labeled graphs associated with neural networks. We conclude with a summary and discussion of the semantics of the cognitive features of our development: memes, themata, qualia, consciousness operators, awareness field.     

Associative relationships between pre-sleep sentence stimuli and reports of mental sleep experience

The aim of this experiment was to study some linguistic relationships between pre-sleep verbal stimuli and contents of reports of mental experience during sleep. In 4 weekly sessions 16 subjects listened before sleep to a sentence stimulus, which was either semantically acceptable (SEM+) or not (SEM-), and were told to retain it for a recall test after awakening; they were awakened once each night during NREM or REM sleep and asked to report their mental experience during sleep. The relationships between the stimulus and the contents of the reports were classified using Clark's (1970) associative rules. Both pre-sleep sentence stimuli were frequently incorporated into contents of NREM and REM reports, without significant differences between the two types of sleep. The SEM+ sentence led prevalently to incorporations through paradigmatic associative relationships, while the SEM- sentence led to incorporations through both paradigmatic and syntagmatic relationships. It appears that all the features of the lexical constituents of the stimulus may be involved in the processing leading to incorporation.     

Interserial and intraserial interference processes in morning recall of mental sleep experience

This study investigated ways in which predictions from interference theory might explain partial or total failure in the morning recall of mental sleep experience (MSE) episodes reported after provoked night awakening. Interference could take place either between the contents of one MSE episode (intraserial interference) or between the contents of different MSE episodes (interserial interference) and would be more likely to affect morning recall where similar (i.e., interrelated) contents in MSEs are involved. If interference were mainly intraserial, in the morning there would be no change in the difficulty of retrieving interrelated contents, whereas, if it were mainly interserial, there would be increased difficulty in the retrieval of such contents. All the possible pairs of reports obtained by awakening 8 subjects four times in REM sleep during each of four experimental nights spaced at weekly intervals were scored for interrelated contents, using Clark's (1970) system of analysis of paired units, which operates by matching linguistic features. Results showed that interrelations in morning report pairs are as frequent as in night report pairs, maintaining the same linguistic characteristics and with little transfer of contents from one MSE to another. Overall these findings are compatible with the hypothesis that interference is primarily intraserial rather than interserial. It is consequently suggested that morning recall failure may be due to temporary blocking of retrieval.     

The electrophysiological correlates of dream recall and nonrecall from stage 2 sleep

This research examined the relationship between cortical activation, defined by electroencephalographic (EEG) measures, and the ability to recall dreams following awakenings from Stage 2 sleep. Period-analyzed EEG data from 40 subjects were examined for the preawakening, postawakening, and preawakening-to-postawakening time intervals. Recall differed from nonrecall at the postawakening and preawakening-to-postawakening periods on measures of muscle activity and time spent in the sigma (12-16 Hz) frequency band. There were no distinctions in recall ability on EEG hemispheric asymmetry measures. Generally, the findings do not support the hypothesis linking increased recall ability to increases in cortical activation prior to awakening. However, the recall groups depicted a different pattern of arousal in their transition from sleep to wakefulness.